Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Dr Tony Balls on July 07, 2021, 07:52:10 am

Title: B+ Quandary
Post by: Dr Tony Balls on July 07, 2021, 07:52:10 am

Working on an amp right now that is giving me lower than expected B+ values and I want to know why.  Here's what I have and what i'm getting:

PT Secondaries: 315 - 0 - 315 VAC
Rectifier: 5AR4
B+ (no tubes) : 444 VDC
B+ (w/ tubes) : 373 VDC

I'd expect to see somewhere in the 420-430 VDC range for the B+ under load, and i've checked with an online calculator to verify that.  Anyone have ideas why my B+ is so low?  Its a cathode biased amp, 2xEL34 push pull.  Started with a 250 ohm shared cathode resistor, and the intent was to tailor that value as needed but I'm trying to get a grip on this B+ first.

Title: Re: B+ Quandary
Post by: sluckey on July 07, 2021, 08:10:54 am

2xEL34 running with cathode bias is a BIG current load. It's perfectly normal for the B+ to drop that much with most PTs. Now if you use a PT that's capable of delivering 500mA, the B+ drop will be much less, but using a PT that big would be insane for this amp.

You can increase the B+ by using a SS rectifier.

Title: Re: B+ Quandary
Post by: Dr Tony Balls on July 07, 2021, 08:34:45 am

Quote from: sluckey on July 07, 2021, 08:10:54 am

2xEL34 running with cathode bias is a BIG current load. It's perfectly normal for the B+ to drop that much with most PTs. Now if you use a PT that's capable of delivering 500mA, the B+ drop will be much less, but using a PT that big would be insane for this amp.

You can increase the B+ by using a SS rectifier.

Thanks, Sluckey.  It must just be something i've never run across before but always good to sanity check with others.  I was planning for a plate current of ~140mA, an the PT is spec'd at 175mA. 

Title: Re: B+ Quandary
Post by: jjasilli on July 07, 2021, 08:53:27 am

Ditto to sluckey.  315VAC x 1.4.4 = 445VDC.  "Electric Guitar Amplifier handbook", by Jack Darr: expect unloaded B+ to drop 10% - 15% under load = 378VDC - 400VDC.  You results are in substantial compliance.


Your tube rectifier - 5AR4 - drops about 30VDC, in the middle of the range for tube rectifiers.  Hence, SS rectification will yield about 403VDC.  Your higher voltage expectations are not achievable with that PT.  Your power supply calculator verification must not have accounted for the specific variables of your PT.

Title: Re: B+ Quandary
Post by: acheld on July 07, 2021, 09:08:14 am

Most comprehensive:  https://el34world.com/Forum/index.php?action=dlattach;topic=20455.0;attach=58985

Easier to view:  https://300guitars.com/articles/rectifier-tube-voltage-drop-chart/

The two don't always agree.   

Title: Re: B+ Quandary
Post by: PRR on July 07, 2021, 11:36:22 am

Those cheat sheets don't really save any time because they are never right.

If you plot the deal on the 5AR4 datasheet you come to very near 370V-375V.